I tend to go through them in waves. I'll remember I have them, and then start using them up for the next few semi-difficult combats, and then start running a little low (at least of the more general-purpose items), and then quit using them for several hours. MAYBE I remember them for the next boss battle.

I'm getting better at it though. But it's still something that bugs me, particularly with my own game. The expendables are supposed to be there to make the first adventure much easier (especially since you can't return to the inn and rest up in mid-adventure). But people are still having a tough time with it. I'm relatively certain that most players who are having a tough time with it aren't taking advantage of their expendable magic items that they start with or find during that first adventure.